f980defe17
* egraph support: rewrite to work in terms of CLIF data structures. This work rewrites the "egraph"-based optimization framework in Cranelift to operate on aegraphs (acyclic egraphs) represented in the CLIF itself rather than as a separate data structure to which and from which we translate the CLIF. The basic idea is to add a new kind of value, a "union", that is like an alias but refers to two other values rather than one. This allows us to represent an eclass of enodes (values) as a tree. The union node allows for a value to have *multiple representations*: either constituent value could be used, and (in well-formed CLIF produced by correct optimization rules) they must be equivalent. Like the old egraph infrastructure, we take advantage of acyclicity and eager rule application to do optimization in a single pass. Like before, we integrate GVN (during the optimization pass) and LICM (during elaboration). Unlike the old egraph infrastructure, everything stays in the DataFlowGraph. "Pure" enodes are represented as instructions that have values attached, but that are not placed into the function layout. When entering "egraph" form, we remove them from the layout while optimizing. When leaving "egraph" form, during elaboration, we can place an instruction back into the layout the first time we elaborate the enode; if we elaborate it more than once, we clone the instruction. The implementation performs two passes overall: - One, a forward pass in RPO (to see defs before uses), that (i) removes "pure" instructions from the layout and (ii) optimizes as it goes. As before, we eagerly optimize, so we form the entire union of optimized forms of a value before we see any uses of that value. This lets us rewrite uses to use the most "up-to-date" form of the value and canonicalize and optimize that form. The eager rewriting and acyclic representation make each other work (we could not eagerly rewrite if there were cycles; and acyclicity does not miss optimization opportunities only because the first time we introduce a value, we immediately produce its "best" form). This design choice is also what allows us to avoid the "parent pointers" and fixpoint loop of traditional egraphs. This forward optimization pass keeps a scoped hashmap to "intern" nodes (thus performing GVN), and also interleaves on a per-instruction level with alias analysis. The interleaving with alias analysis allows alias analysis to see the most optimized form of each address (so it can see equivalences), and allows the next value to see any equivalences (reuses of loads or stored values) that alias analysis uncovers. - Two, a forward pass in domtree preorder, that "elaborates" pure enodes back into the layout, possibly in multiple places if needed. This tracks the loop nest and hoists nodes as needed, performing LICM as it goes. Note that by doing this in forward order, we avoid the "fixpoint" that traditional LICM needs: we hoist a def before its uses, so when we place a node, we place it in the right place the first time rather than moving later. This PR replaces the old (a)egraph implementation. It removes both the cranelift-egraph crate and the logic in cranelift-codegen that uses it. On `spidermonkey.wasm` running a simple recursive Fibonacci microbenchmark, this work shows 5.5% compile-time reduction and 7.7% runtime improvement (speedup). Most of this implementation was done in (very productive) pair programming sessions with Jamey Sharp, thus: Co-authored-by: Jamey Sharp <jsharp@fastly.com> * Review feedback. * Review feedback. * Review feedback. * Bugfix: cprop rule: `(x + k1) - k2` becomes `x - (k2 - k1)`, not `x - (k1 - k2)`. Co-authored-by: Jamey Sharp <jsharp@fastly.com>
Cranelift Code Generator
A Bytecode Alliance project
Cranelift is a low-level retargetable code generator. It translates a target-independent intermediate representation into executable machine code.
For more information, see the documentation.
For an example of how to use the JIT, see the JIT Demo, which implements a toy language.
For an example of how to use Cranelift to run WebAssembly code, see Wasmtime, which implements a standalone, embeddable, VM using Cranelift.
Status
Cranelift currently supports enough functionality to run a wide variety of programs, including all the functionality needed to execute WebAssembly (MVP and various extensions like SIMD), although it needs to be used within an external WebAssembly embedding such as Wasmtime to be part of a complete WebAssembly implementation. It is also usable as a backend for non-WebAssembly use cases: for example, there is an effort to build a Rust compiler backend using Cranelift.
Cranelift is production-ready, and is used in production in several places, all within the context of Wasmtime. It is carefully fuzzed as part of Wasmtime with differential comparison against V8 and the executable Wasm spec, and the register allocator is separately fuzzed with symbolic verification. There is an active effort to formally verify Cranelift's instruction-selection backends. We take security seriously and have a security policy as a part of Bytecode Alliance.
Cranelift has three backends: x86-64, aarch64 (aka ARM64), and s390x (aka IBM Z). All three backends fully support enough functionality for Wasm MVP, and x86-64 and aarch64 fully support SIMD as well. On x86-64, Cranelift supports both the System V AMD64 ABI calling convention used on many platforms and the Windows x64 calling convention. On aarch64, Cranelift supports the standard Linux calling convention and also has specific support for macOS (i.e., M1 / Apple Silicon).
Cranelift's code quality is within range of competitiveness to browser JIT engines' optimizing tiers. A recent paper includes third-party benchmarks of Cranelift, driven by Wasmtime, against V8 and an LLVM-based Wasm engine, WAVM (Fig 22). The speed of Cranelift's generated code is ~2% slower than that of V8 (TurboFan), and ~14% slower than WAVM (LLVM). Its compilation speed, in the same paper, is measured as approximately an order of magnitude faster than WAVM (LLVM). We continue to work to improve both measures.
The core codegen crates have minimal dependencies and are carefully written to handle malicious or arbitrary compiler input: in particular, they do not use callstack recursion.
Cranelift performs some basic mitigations for Spectre attacks on heap bounds checks, table bounds checks, and indirect branch bounds checks; see #1032 for more.
Cranelift's APIs are not yet considered stable, though we do follow semantic-versioning (semver) with minor-version patch releases.
Cranelift generally requires the latest stable Rust to build as a policy, and is tested as such, but we can incorporate fixes for compilation with older Rust versions on a best-effort basis.
Contributing
If you're interested in contributing to Cranelift: thank you! We have a contributing guide which will help you getting involved in the Cranelift project.
Planned uses
Cranelift is designed to be a code generator for WebAssembly, but it is general enough to be useful elsewhere too. The initial planned uses that affected its design were:
- Wasmtime non-Web wasm engine.
- Debug build backend for the Rust compiler.
- WebAssembly compiler for the SpiderMonkey engine in Firefox (currently not planned anymore; SpiderMonkey team may re-assess in the future).
- Backend for the IonMonkey JavaScript JIT compiler in Firefox (currently not planned anymore; SpiderMonkey team may re-assess in the future).
Building Cranelift
Cranelift uses a conventional Cargo build process.
Cranelift consists of a collection of crates, and uses a Cargo
Workspace,
so for some cargo commands, such as cargo test, the --all is needed
to tell cargo to visit all of the crates.
test-all.sh at the top level is a script which runs all the cargo
tests and also performs code format, lint, and documentation checks.
Log configuration
Cranelift uses the log crate to log messages at various levels. It doesn't
specify any maximal logging level, so embedders can choose what it should be;
however, this can have an impact of Cranelift's code size. You can use log
features to reduce the maximum logging level. For instance if you want to limit
the level of logging to warn messages and above in release mode:
[dependency.log]
...
features = ["release_max_level_warn"]
Editor Support
Editor support for working with Cranelift IR (clif) files: